The Oxford nuclear orientation group has a worldwide reputation for high quality research in low-energy nuclear and hyperfine interaction physics. In recent years it has taken a primary role in the development of the demanding multi-disciplinary technique of on-line low-temperature nuclear orientation (OLNO) in which angular properties of nuclear decay processes are studied through the detection of emissions from sources of isotopes polarised at millikelvin temperatures. High degrees of nuclear polarisation of radioactive isotopes are achieved following implantation into ferromagnetic metal foils. The experiments are carried out 'on-line' at accelerators, thus giving experiments on polarised isotopes with half-lives as short as 30 ms. Using a polarised source, angular dependent properties are observable, with no coincidence requirement, by measuring emissions at a range of angles with respect to the polarisation axis. Such experiments give information on aspects of decay mechanisms and nuclear ground and excited states which is otherwise difficult or impossible to obtain. The Oxford group is unique in the UK and one of the few worldwide who have the expertise to generate this kind of result.
On-line OLNO study of nuclei ever further from stability began at the Daresbury NSF in the early nineteen eighties. Later in that decade the Principle Investigator acted as spokesman for a collaboration involving six European countries in setting up the NICOLE OLNO system at the ISOLDE isotope production facility at CERN. The group thus became the first from the UK to make ISOLDE a major experimental base. The ex-NSF on-line orientation facility was moved by the Oxford group to the OSIRIS fission fragment separator at Studsvik, Sweden in 1994, where it has since been operated very successfully. At each of these facilities the group has a very strong record of getting proposals accepted by the international proposal committee and of extracting significant physics from the experimental results. The Oxford group has collaborated extensively with other experimentalists and theoreticians.
The early on-line experiments gave details of nuclear excited state structure and decay which formed critical tests of nuclear models. They yielded precise nuclear magnetic dipole moments, a sensitive measure of the single particle make-up of the oriented isotope. Published work made significant contribution to understanding of deformation in the light bromine isotopes, and of intruder states in proton rich antimony, tellurium, iodine and xenon isotopes, mid-shell between the neutron closed shells at ‘magic numbers’ N=50 and N=82. The work at Studsvik has concentrated on neutron rich isotopes of the same elements close to the ‘double magic’ isotope of tin, 132Sn (Z=50, N=82). At CERN the focus initially was on magnetic moments and shape evolution in the heavier elements mercury, gold, platinum and iridium and more recently on magnetic moments in nickel and copper close to ‘double magic’ 68Ni (Z=28, N=40).
The group has made significant innovations of experimental method, most notably a long association with use of NMR (nuclear magnetic resonance) in combination with orientation to give precise nuclear dipole moments. It was the first to recognise the importance of time dependent aspects of on-line orientation as the nuclei cool following implantation and initiated pulsed on-line experiments in the Time–Resolved Nuclear Orientation (TROLNO) technique.
The alliance of the best experiments with the best theory has been of increasing importance in recent years. We have achieved a significant series of collaborative works on nuclear magnetic moments of isotopes close to double magic shell closure, both near 132Sn and 68Ni and have collaborated in their interpretation with I.S.Towner and B.A.Brown, world class theorists in this type of physics. Our experimental results have critically tested theory regarding the mechanisms responsible for the quenching of spin and orbital contributions to nuclear magnetism in finite nuclei, as compared with free nucleons. Explanation in terms of meson exchange currents and core polarisation, previously tested only in light nuclei and very heavy (Pb) nuclei, has been shown to have impressive consistency across the whole periodic table. Two recent Physical Review Letters have resulted from this work.
In a new initiative aimed at quite different physics the group has worked hard in recent years exploring the study of the angular distribution of particle emission from oriented nuclei. This extension of technique is made possible by the very thin samples which are produced by ion implantation which minimises in-source scattering of the emitted particles (electrons, positrons, alpha particles and most recently protons and neutrons). Semiconductor detectors have been set up operating inside the cryostat at 4 K to detect the emitted charged particles that, unlike gamma rays and neutrons, do not penetrate the dewar of the dilution refrigerator. Through experiments involving the more ‘established’ electron and alpha particle emission, we have obtained useful results and gained valuable experience in how to make accurate and reliable measurements of this kind. However the initiative was prompted by the rapid new field of proton decay research close to the limits of nuclear stability, and the related fields of beta-delayed proton and neutron emission.
The systematic study of drip-line decay processes has matured to the stage at which questions of detail beyond the basic energetics are being addressed. Mapping the limits of nuclear existence - the 'drip lines' - is a vital input to understanding the production of chemical elements in astrophysical production processes. Strong input was given by the Edinburgh group working initially at the N.S.F. Daresbury and more recently at Argonne and Oak Rodge National Laboratories. However these experiments are restricted to energy and lifetime measurements - they have no access to angular properties of these important emission processes. This is the area in which on-line nuclear orientation can play a vital role and on which this proposal is based. This proposal is the first to be devoted to this new field of opportunity.
There is a series of open questions in the theory of both beta-delayed and direct proton emission which can be approached by angular distribution studies, as outlined in the proposal. The OLNO technique, at ISOLDE, is the only method to offer quantitative angular distribution measurements of these phenomena. At Studsvik we have the opportunity to explore the same features of beta-delayed neutron emission, another completely novel experiment. The strong interest shown in our proposed experiments at a meeting devoted entirely to proton emission at Oak Ridge in October 1999 and a recent invitation to address a planned workshop at Trento in November 2000 have given peer demonstration of the timeliness of work in this direction.
Building upon our electron and alpha experience the group recently at ISOLDE took the first steps in this new direction, making, as a successful pilot study, the first ever measurement of the angular distribution of beta-delayed protons. With an on-line polarised source of the isotope 118Cs implanted in iron it proved possible to detect the low intensity proton emission in the presence of far stronger beta particle flux and clear anisotropy of the proton emission was detected. Furthermore, at Studsvik in October 2000, we established that the beta-delayed neutron process was open to study by demonstrating that neutrons from a source in the dilution refrigerator could be readily detected against ambient background from the nearby reactor.
Development of this novel field, of proven viability, takes the nuclear orientation method into a new area and towards physics of great topical and general interest.